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Date:   Sat, 25 Mar 2023 09:25:36 -0700
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Subject: Re: [PATCH] x86: Disable kexec for TDX guests
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20230325160128.21857-1-kirill.shutemov@linux.intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20230325160128.21857-1-kirill.shutemov@linux.intel.com>
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On 3/25/23 09:01, Kirill A. Shutemov wrote:
> The last item is tricky. TDX guests use ACPI MADT MPWK to bring up
> secondary CPUs. The mechanism doesn't allow to put a CPU back offline if
> it has woken up.
...
> +int arch_kexec_load(void)
> +{
> +	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
> +		pr_warn_once("Disable kexec: not yet supported in TDX guest\n");
> +		return -EOPNOTSUPP;
> +	}
> +
> +	return 0;
> +}

So, let's put all this together:

1. TDX implementations use MADT for wakeup exclusively right now (but
   are not necessarily _required_ to do so forever)
2. MADT doesn't support CPU offlining
3. kexec() requires offlining

Thus, current TDX implementations can't support TDX guests.  This
*doesn't* say that TDX will always use the MADT for wakeups.

Yet, the check you have here is for TDX and *not* for the MADT.

That seems wrong.

Let's say SEV or arm64 comes along and uses the MADT for their guests.
They'll add another arch_kexec_load(), with a check for *their* feature.

This all seems like you should be disabling kexec() the moment the MADT
CPU wakeup is used instead of making it based on TDX.